Computed tomography (CT)imaging arrays typically comprise large arrays of photosensor devices coupled through an optical coupling layer to a scintillator in which the incident radiation (such as x-rays) to be detected by the imager is absorbed. Light photons generated in the scintillator as a result of the absorption of incident radiation pass to the photodetector array and are in turn absorbed by the photosensor, resulting in the accumulation of charge on the photosensor that corresponds to the flux of light photons; reading the charge accumulated on respective photosensors provides a measure of the intensity of the incident x-rays and the relative position on the array at which the x-ray radiation was absorbed.
In conventional CT arrays, photosensors are arranged in a one-dimensional array with each photosensor being directly coupled to readout electronics (in a one-dimensional array, the photosensor are aligned in a pattern along one axis). In volumetric CT devices the photosensor arrays typically comprise photosensor array blocks in which photodiodes are arranged in a two-dimensional pixel array (that is, disposed in a pattern along two axes). These photosensor arrays typically comprise a block of semiconductive material in which respective contact pads are formed along one surface of the array block and an electrode is disposed along an opposite surface of the array block. The respective photosensors in the block correspond to each contact pad; a bias voltage is applied by the back electrode to the respective photosensors in the array and results in a depletion region being formed in the semiconductive material surrounding the contact pads. Charge generated in the photodiode as a result of the absorption of light photons from the scintillator is collected at respective pixel contact pads and processed by the readout electronics to generate the imager output signal.
It is desirable that the photosensor array in a CT imager array exhibit low noise, low cross-talk between photodiodes, and a high degree of linearity (that is, respective photosensors developing corresponding amounts of charge in response to the same intensity of incident radiation on the imager). For example, to minimize cross-talk between photodiodes, reflective or light absorptive material is disposed around portions of the scintillator overlying respective pixels in order to reduce scattering of light photons within the scintillator to pixels other than the pixel underlying the portion of the scintillator in which the light photons were formed in response to the absorption of the incident radiation.
In addition, as new two-dimensional arrays are developed for use in volumetric C T, it is thought that the light-transmissive optical coupling layer in convention CT type imager arrays will present a problem with respect to cross-talk and linearity of the respective photodiodes. Optical photons generated in one portion of the scintillator overlying a particular pixel can be scattered and pass through the optical coupling layer into an adjoining pixel; this passage presents problems of increased cross-talk in the array, that is, detection in photodiodes of light absorbed in portions of the scintillator other than that portion of the scintillator overlying the pixel, which reduces the spatial resolution of the array.
Further, the absorption of photons in the region between the diodes can affect the linearity of the array as the amount of charge collected on a particular diode is a function of the voltage across the diode (the bias voltage across the diode affects the extent of the depletion region surrounding the contact pad and hence the sensitive, or photoactive, region of a given photodiode). Thus, the size of depletion region of a given photodiode is affected by signal strength as the greater the number of incident optical photons the greater the amount of charge collected, and thus the greater the reduction in the size of the depletion region. Charge not collected by one photodiode in which the depletion region has become smaller due to reduced bias voltage may be lost through recombination; this loss results in non-linearity in the array. Such absorption of photons in the region between the diodes can result from: the scintillator being larger than the underlying diodes; optical photons leaving the scintillator at an angle such as to hit the region between the diodes; and, scattering of optical photons in the optical coupling layer between the scintillator and the photosensor array.
It is thus an object of this invention to provide a CT photosensor array having low cross-talk and high linearity of photosensor devices in the array.
It is a further object of this invention to provide a low noise photosensor array,